The Swiss will also play mixed doubles; the American believes the injury isn't serious. (AP)

PERTH, Australia (AP) — Roger Federer continued preparation for his Australian Open title defense with a 6-3, 7-6 (8) win over Karen Khachanov at the Hopman Cup, helping give Switzerland a 2-0 win over Russia.

Federer's playing partner Belinda Bencic beat Anastasia Pavlyuchenkova 6-1, 3-6, 6-3 in a later women's singles match Tuesday to give Switzerland its second consecutive win in Group B.

The 36-year-old Federer had his service broken by the 21-year-old Khachanov in the opening game, but rebounded to break back immediately and won the opening set in 29 minutes.

Khachanov matched Federer impressively with backhand winners in the second set before a back-and-forth tiebreaker. Federer saved a set point in the tiebreaker and won on his third match point after a successful challenge.

"I had to work hard, experience helps sometime," said Federer, who beat Rafael Nadal in the Australian Open final last year. "It's good to be free-swinging and young, like he is. I remember those days ..."

The win came before a record crowd of 13,943 at Perth Arena, an attendance which eclipsed one of Federer's matches last year in the Western Australia state capital.

Earlier, the United States beat Japan 2-1 after an illness withdrawal by one of the Japanese players. But the win may come at a cost after Jack Sock injured his hip.

Sock slipped on the court during the first set of his match against Yuichi Sugita, grabbing his hip. He was forced to retire at 1-1 in the second set after losing the first set 7-6 (1).

Hopman Cup director Paul Kilderry said he thinks Sock will be OK for the unbeaten U.S. team's important third game.

"He has had some treatment on it and he feels he'll be good to go," Kilderry said. "He said if felt uncomfortable out there, so I think he was just a little nervous about doing more damage."

The U.S. team had already led 1-0 after Japan's female player, Naomi Osaka, withdrew with an unspecified illness, handing CoCo Vandeweghe a 6-0, 6-0 win.

The Americans were also awarded the Fast4 mixed doubles match 4-0, 4-0 because Osaka was not able to compete.

Osaka's place in both matches was taken by 19-year-old local player Maddison Inglis, who was woken up at 8:30 a.m. by tournament officials who told her she would be required to play.

The U.S. and Switzerland play on Thursday in a match that will decide which team advances from the group to Saturday's final.